Polyolefin foams and methods of manufacturing polyolefin foam rods, planks, and sheets are well known in the art. See, e.g., U.S. Pat. Nos. 5,462,974 and 5,667,728, the entire disclosures of which are incorporated herein by reference. One of the most common polyolefins used to produce foam is polyethylene and, specifically, low density polyethylene (LOPE). While LDPE possesses a number of beneficial physical and chemical properties when used to produce a foamed product, one disadvantage is that the physical blowing agents commonly used (hydrocarbons, chlorinated hydrocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, or combinations thereof) can lead to the formation of smog, have high ozone depletion potential or global warming potential, and/or can be hazardous air pollutants. Further, the long curing process and flammability associated with hydrocarbons have generated a new interest in the development of new technologies to utilize non-flammable physical blowing agents such as carbon dioxide and nitrogen. Thus, the use of hydrocarbons and halogenated hydrocarbon blowing agents for preparing polymeric foams is not preferred environmentally and imposes many limitations on the manufacturing process, thus complicating and significantly increasing the cost of manufacture.
However, polyolefin traditionally does not exhibit favorable foaming behavior when physical blowing agents (such as carbon dioxide) are used. Particularly, when used as a physical blowing agent in traditional polyolefin (such as LPDE) foam processes, carbon dioxide produces non-descript masses of polymeric material or otherwise poor quality thermoplastic foams that collapse. It is believed that the lack of polymer-gas compatibility and limited solubility of carbon dioxide within the molten thermoplastic extrudate lead to the production of an uncontrollably high level of open cells in the foam structure as the thermoplastic/blowing agent combination exits the die. Additionally, even if the resultant foams have a visible foam structure, they tend to collapse quickly due to the relatively high permeability of carbon dioxide relative to air (i.e., the cells can collapse due to the partial vacuum created by the rapid escape of the carbon dioxide from the cells) and become useless for most practical applications within 24 hours of manufacture.
The presently disclosed subject matter is directed to a method of foaming polyolefin using a physical blowing agent. Specifically, when the polyolefin is blended with an acrylated epoxidized fatty acid (such as acrylated epoxidized soybean oil) and a physical blowing agent (such as carbon dioxide) is used, the foamability of the polyolefin dramatically increases.